Discrete ManufacturingFree referencePDF, Excel, Word exportMachining

Tap Drill Size Chart for Metric, UNC and UNF Threads

This chart answers the question every machinist asks before a tapping operation: which drill goes under this tap. It covers common metric coarse, metric fine, UNC and UNF sizes with the drill designation and its diameter in millimetres, so you can search once and move on.

Download:
ThreadSeriesPitch / TPIMajor dia (mm)Tap drillDrill dia (mm)
M3 x 0.5
Metric coarse
0.5 mm
3
2.5 mm
2.5
M4 x 0.7
Metric coarse
0.7 mm
4
3.3 mm
3.3
M5 x 0.8
Metric coarse
0.8 mm
5
4.2 mm
4.2
M6 x 1
Metric coarse
1.0 mm
6
5.0 mm
5
M8 x 1.25
Metric coarse
1.25 mm
8
6.8 mm
6.8
M10 x 1.5
Metric coarse
1.5 mm
10
8.5 mm
8.5
M12 x 1.75
Metric coarse
1.75 mm
12
10.2 mm
10.2
M14 x 2
Metric coarse
2.0 mm
14
12.0 mm
12
M16 x 2
Metric coarse
2.0 mm
16
14.0 mm
14
M20 x 2.5
Metric coarse
2.5 mm
20
17.5 mm
17.5
M24 x 3
Metric coarse
3.0 mm
24
21.0 mm
21
M8 x 1
Metric fine
1.0 mm
8
7.0 mm
7
M10 x 1.25
Metric fine
1.25 mm
10
8.8 mm
8.8
M12 x 1.25
Metric fine
1.25 mm
12
10.8 mm
10.8
M12 x 1.5
Metric fine
1.5 mm
12
10.5 mm
10.5
M16 x 1.5
Metric fine
1.5 mm
16
14.5 mm
14.5
M20 x 1.5
Metric fine
1.5 mm
20
18.5 mm
18.5
#4-40
UNC
40 TPI
2.845
#43 (0.0890 in)
2.261
#6-32
UNC
32 TPI
3.505
#36 (0.1065 in)
2.705
#8-32
UNC
32 TPI
4.166
#29 (0.1360 in)
3.454
#10-24
UNC
24 TPI
4.826
#25 (0.1495 in)
3.797
1/4-20
UNC
20 TPI
6.35
#7 (0.2010 in)
5.105
5/16-18
UNC
18 TPI
7.938
F (0.2570 in)
6.528
3/8-16
UNC
16 TPI
9.525
5/16 in (0.3125 in)
7.938
7/16-14
UNC
14 TPI
11.113
U (0.3680 in)
9.347
1/2-13
UNC
13 TPI
12.7
27/64 in (0.4219 in)
10.716
9/16-12
UNC
12 TPI
14.288
31/64 in (0.4844 in)
12.304
5/8-11
UNC
11 TPI
15.875
17/32 in (0.5313 in)
13.495
3/4-10
UNC
10 TPI
19.05
21/32 in (0.6563 in)
16.669
7/8-9
UNC
9 TPI
22.225
49/64 in (0.7656 in)
19.446
1-8
UNC
8 TPI
25.4
7/8 in (0.8750 in)
22.225
#10-32
UNF
32 TPI
4.826
#21 (0.1590 in)
4.039
1/4-28
UNF
28 TPI
6.35
#3 (0.2130 in)
5.41
5/16-24
UNF
24 TPI
7.938
I (0.2720 in)
6.909
3/8-24
UNF
24 TPI
9.525
Q (0.3320 in)
8.433
7/16-20
UNF
20 TPI
11.113
25/64 in (0.3906 in)
9.922
1/2-20
UNF
20 TPI
12.7
29/64 in (0.4531 in)
11.509
3/4-16
UNF
16 TPI
19.05
11/16 in (0.6875 in)
17.463

Drills are the common cutting-tap choices giving roughly 65 to 75 percent thread engagement in typical steels. Forming (roll) taps need a larger drill, so use the tap maker's chart for those. Always confirm against your tap manufacturer's data and the part drawing.

Keep tooling data consistent in your ERP

If you want thread, drill and tap data tied to your item and routing records, tell us what you run and we will suggest an approach.

Optional. The tool and every export work without this. No spam.

How to use the chart

Type the thread into the search box, for example M8, 1/4-20 or UNF, and read the tap drill from the matching row. The last column gives the drill diameter in mm, which is what most CNC tool tables and ERP tool masters want. Number and letter drills are listed with their inch size in brackets so you can check them against your drill index. Export the filtered rows to Excel or PDF to pin a copy at the machine or to load into a tooling list.

  • Search by thread, series or drill name
  • Use the mm column when setting up tool tables
  • Export a filtered subset for a job traveller

Where the numbers come from

For a 60 degree thread the tap drill is the major diameter minus the pitch times a factor tied to the thread engagement you want. Metric sizes follow the simple rule major minus pitch, which yields roughly 77 percent engagement for M10 x 1.5 with an 8.5 mm drill. Inch sizes use the nearest standard fractional, number or letter drill to the target diameter. Because drills come in fixed steps the real engagement lands within a few percent of the target, which is acceptable because 60 to 75 percent engagement gives nearly the same holding strength with far lower tapping torque.

Reading engagement and choosing a size

Higher engagement means a stronger thread on paper but a much harder tap job and more tap breakage. In steel, 65 to 75 percent is the normal target. In soft aluminium or plastics a smaller drill with more engagement is fine, while in stainless, titanium and nickel alloys a slightly larger drill that reduces torque is often the better choice. If a drawing calls out a thread class and minor diameter limits, the drawing wins over any chart.

Common mistakes

The most frequent error is using a metric drill for an inch thread because the sizes look close, for example 6.8 mm versus the 6.9 mm needed for 5/16-24. Another is using a cutting-tap drill with a forming tap, which produces a thread that is too tight or snaps the tap. Finally, remember that the drilled hole needs enough depth beyond the full thread length for the chamfer lead and chip space when tapping blind holes.

  • Do not mix metric and inch drills
  • Forming taps need a different drill
  • Allow extra hole depth for the tap lead
  • Check coating and hole depth in blind holes

Frequently Asked Questions

What size drill do I use for an M8 tap?

For a standard M8 x 1.25 coarse thread use a 6.8 mm drill. For the M8 x 1 fine thread use a 7.0 mm drill. Both give about 70 to 77 percent engagement with a normal cutting tap in steel and similar materials.

What is the tap drill for 1/4-20?

The usual tap drill for 1/4-20 UNC is a number 7 drill at 0.2010 inch, or 5.105 mm. For 1/4-28 UNF the usual choice is a number 3 drill at 0.2130 inch, or 5.410 mm.

Is this chart valid for forming taps?

No. Forming or roll taps displace metal instead of cutting it, so they need a larger drill, usually close to the pitch diameter. Use the chart from the tap manufacturer, because the correct size depends on the tap and the material.

How much thread engagement do I need?

Sixty to seventy-five percent is normal for steel. Beyond that, strength gain is small and tap load climbs fast. Soft materials may justify more, and tough alloys often justify less to protect the tap.

If your tool masters and routings live in an ERP, we can help keep thread, drill and tap data consistent with your item and operation records.

Talk to an ExpertExplore More Free Tools

Related Tools

Discrete Manufacturing

Thread Pitch Chart (Metric, UNC, UNF)

Reference of thread pitch in mm, threads per inch and approximate thread depth for common metric, UNC and UNF sizes.

Discrete Manufacturing

Tapping Torque Calculator

Estimates cutting tap torque, thread engagement and spindle power from thread size, tap drill diameter, material and tap condition.

Discrete Manufacturing

Thread Milling G-Code Generator

Fill in thread size, pitch, tool path radius and feed to generate a Fanuc-style helical thread milling program for an internal right-hand thread.

Discrete Manufacturing

Drill Point Length Calculator

Calculates drill point length for common point angles, the Z depth to program for a full-diameter hole, spindle rpm, feed and drilling time.

More Discrete Manufacturing tools

See all
Back to All Tools